1. Field of the Invention
The present invention relates to a motor-operated power steering apparatus of the rack-and-pinion type, and more particularly to an improvement in the structure and arrangement of a motor control device in the motor-operated power steering apparatus.
2. Description of the Relevant Art
Conventional motor-operated power steering apparatus of the rack-and-pinion type include an electric motor disposed on or in a gear case for generating assistive steering power and a torque detector disposed near a gear mechanism in the gear case. A control device for controlling the motor is housed in a unit case attached to a vehicle body separately from the gear case. The motor and the torque detector are electrically connected to the control device in the unit case by means of lead wires. Since the unit case and the gear case are independent of each other, they have to be attached separately to the vehicle body. The lead wires between the separate unit case and the gear case are long and the arrangement of the lead wires tends to be complex, causing an electric power loss. Inasmuch as the long lead wires are liable to pick up noise from other electric devices, the reliability of the apparatus is reduced. In addition, the cost of manufacture is high as the assembly procedure is time-consuming.
To solve the above problems, there has been proposed a motor-operated power steering apparatus in which a motor control device including a control circuit and a driver circuit is accommodated in a gear case to make the apparatus smaller and more compact, and to reduce the cost, the electric power loss, and the noise susceptibility, as disclosed in Japanese Laid-Open Patent Publication No. 61-119467.
With the electric control device housed in the gear case, however, the heat generated by the driver circuit when large currents flow through power transistors which drive the motor adversely affects the other circuit components situated near the driver circuit. Where the gear case is located in an engine compartment that houses an engine and various heat generating parts, the control circuit is adversely affected by a hot atmosphere produced therearound.